This invention relates to an improved screw for use in an extruder for working a wide range of solid materials into a substantially homogeneous, molten state suitable for formation into a multitude of desired shapes by extrusion or injection into a die, mold, or casting form. More particularly, the improved screw of the present invention is most readily incorporated into what is known as a single screw extruder.
Extruder screws are used for transforming solid materials into a molten mixture for subsequent use in a mold or die via extrusion or injection. Materials that are used for this purpose include a range of plastic materials, as well as various metal or thixotropic materials. The screw consists of a generally cylindrical body with at least one helical thread formed thereon. The outer diameter of the thread is known as the main diameter of the screw, and the areas between the main diameters are known as channels of the screw. The opposing ends of the screw are known as the feed end, where the solid material is introduced, and the discharge end, where the molten material is delivered.
Extrusion, injection molding or blow molding with a single screw extruder, includes feeding the solid material in pellet, chip, powder, or flake form to the feed end of the extruder through a hopper or similar device mounted on an opening of a heated cylindrical barrel. The extruder screw is rotatably mounted and received in the barrel. The screw thread maintains a minimum clearance to the barrel and the material is moved downstream through the channel from the feed end to the discharge end by forces exerted by the rotation of the screw. The solid material fed into the screw channel is compacted into a solid plug or solid bed and the solid bed melts as it travels down the screw channel. The solid bed melts in at least two ways, including by shear melting and conductive melting. Shear melting is caused by the forces exerted by the screw on the solid bed or melt pool. Conductive melting occurs from the transfer of heat from the barrel heaters and when cooler solid pieces of the material come in contact with the hotter liquid melt pool. While these melting types encountered in extruder screw operation occur in most materials, shear melting is generally more common in plastic materials, while conductive melting is generally more common in metal or thixotropic materials.
The molten material is collected by the wiping action of the thread into a melt pool. The melt pool gradually increases as the solid bed gradually melts, eventually occupying the entire screw channel. The best extrusion and injection results are achieved when only molten material is delivered at a desired temperature at the discharge end of the screw. Molten polymers, however, have a very high viscosity and a large amount of heat is generated in the melt pool due to shearing of the melt pool by the rotation of the screw. Thus, the melt pool becomes hotter as it travels down the screw channel and often becomes undesirably hot by the time it reaches the discharge end. Increased heat transfer from the molten material in the melt pool to the solid material in the solid bed is highly desirable in order to reduce the temperature of the molten material discharged from the extruder, increase melting capacity of the extruder and increase the energy efficiency of the extrusion process.
The U.S. Pat. No. 3,487,503 shows an extruder that uses pegs machined into the channel closest to the discharge end of the screw to promote breaking up the solid bed. The U.S. Pat. No. 4,173,417 shows an extruder screw that also increases heat transfer from the melt pool to the solid bed by introducing a second thread that creates sub-channels within the screw channel, but this induces back flow of the solid materials in the opposite direction of the screw rotation.
Extruder screws of the type utilizing sub-channels are typically divided into three sections along the length of the screw. The first section is a feed section having a constant root diameter, where the solid material is introduced to the screw. The second section is the taper section, where the area in which the solid bed travels is gradually reduced by reducing the depth of the channel and where the majority of the melting of the solid occurs. It is in this section where a portion of the screw channel defined by the thread is divided into two sub-channels by a second thread to define a xe2x80x9cbarrier sectionxe2x80x9d. The third section is the metering section, which is similar to the feed section in that it has a constant root diameter, and which delivers the molten material in a constant amount for extrusion or injection. The thread that begins in the feed section and continues into the taper section is known as the wiping thread or main flight. It is this thread that has the minimum clearance to the barrel, and provides the force for moving the material down the length of the extruder screw. The thread that divides the channel into sub-channels is known as a barrier thread because it acts as a barrier that prevents solid particles from passing between the sub-channels. The difference in the diameters of the barrier thread and the wiping thread is known as the barrier clearance.
The U.S. Pat. No. 4,405,239 (the ""239 patent) shows a screw with an energy transfer section between the taper and metering sections. The energy transfer section has a barrier thread that creates a sub-channel within the screw channel. The screw allows the solids particles and the melt pool to flow in a single direction by alternating which thread is used as the wiping threadxe2x80x94the thread with the minimum barrel clearance that moved the material towards the feed end. The depth of each of the sub-channels is varied as in the prior art so as to promote the flow of materials from one sub-channel to the other. This depth variation occurs throughout the length of the sub-channels. As the depth in one sub-channel increases, the other decreasesxe2x80x94a pattern that is repeated throughout the length of the screw""s energy transfer section. In addition, the clearances of the threads are interrupted in relation to the variation in sub-channel depth to increase the back flow noted above. At the point where the depth of the channels is reversed, the threads diameters are also reversed, such that the wiping thread""s clearance is reduced so that the wiping thread becomes the barrier thread, and the barrier thread""s clearance is increased so that the barrier thread becomes the wiping thread. The point at which the threads reverse or convert is the beginning of a barrier section. The thread clearances do not vary in the undulating manner of the sub-channel depth. Instead, there is a quick drop-off from the minimum barrel clearance of the wiping thread to the lesser clearance of the barrier thread, or vice versa. Farther downstream another reversal takes place, defining another barrier section, and the threads resume their original functions. These thread reversals or conversions take place throughout the length of the energy transfer section.
The ""239 patent also discloses barrier sections that are identical both in length and barrier clearance, that is, the clearance between the wiping thread and the barrier thread is constant throughout the screw""s energy transfer section such that the size of the particles that could pass between the sub-channels is constant throughout the length of the screw""s energy transfer section. Constant length and clearance sections have inherent limitations in promoting conductive melting because while mixing is promoted, constant clearances and lengths allow the same size material to pass regardless of its location in the energy transfer section. The refining or dispersion of the melt can be improved by varying the length and clearance of the energy transfer sections so as to only allow smaller and smaller particles to move down the length of the screw. Improvements in the efficiency of conductive melting without sacrificing the flow rate of the materials through the screw remains a continuing goal of extruder screw design. It remains very desirable to increase conductive melting without sacrificing the flow rate or energy efficiency of the screw.
It is an objective of this invention to achieve a gradual increase in the refining or dispersion of the melt by increased shear on the melt as it moves downstream over the clearances without sacrificing the flow rate through the extruder screw.
It is a further objective of the invention to achieve greater conductive melting in an extruder screw caused by greater mixing of the unmelted solids with the melt stream regardless of the type of solids being processed.
The present invention accomplishes the objectives noted above by making a novel improvement upon the sub-channel depth variation methodology utilized in the U.S. Pat. No. 4,405,239. The sub-channels depth and the thread clearances both vary as in the ""239 patent. The present invention, however, recognizes that greater conductive melting can be accomplished by forcing larger solid particles to remain in the melt stream and not be moved farther downstream until reduced in size. It further recognizes that a flow rate reduction may result by forcing these larger particles to remain in the barrier sections for a longer period of time. The present invention will prove particularly useful for processing those materials, such as metals or thixotropic materials, where the majority of melting that occurs is by conductive melting rather than by shear melting. The present invention will also help in continuing to ensure that only liquid material is delivered to the meter section of the extruder screw.
The prior art barrier sections and barrier clearances remain constant throughout the length of the screw""s energy transfer section. The present invention""s improvement lies in gradually reducing the barrier clearances while proportionately increasing the length of the barrier sections. More specifically, the barrier thread""s barrier clearance reduction is inversely proportional to the increase in the length of the barrier section. The length of the barrier section is the distance along the screw between the points where the barrier thread and the wiping thread reverse functions. The number of thread turns, or the distance along the screw, it takes to complete the reversal increases through each section. Larger particles may pass into the first barrier section, but will be too large to enter the next section. Larger particles will not be able to pass through the barrier clearance into the other sub-channel when the sub-channel area decreases, which increases the conductive melting of these particles in the melt pool of that section. Because larger particles are allowed into and remain longer in the barrier section, the overall length of the barrier section is increased to allow the particles longer time to traverse the length of the barrier section and again increase the likelihood of conductive melting. By increasing the length of the barrier sections in an inverse proportion to the reduction in the sub-channel clearances, the flow rate of the screw is not compromised.
The barrier clearance, then, actually tapers or reduces along the length of the extruder screw energy transfer section, in a similar manner as the channel depth tapers in the taper section. The process is repeated through each successive barrier section, until only liquid material passes to the meter section of the extruder screw. As a result of these improvements, better conductive melting is accomplished, the refinement and dispersion of the melt is increased, and more types of solid material may be used, whilst the flow and efficiency of the extruder screw remain unaffected.